1. Field of The Invention
The present invention relates to a magnetic recording medium, specifically to a coated type ferrite magnetic recording medium whose reproduction output is high in a short wave band.
2. Description of Prior Art
Generally, a coated type magnetic recording medium is composed of a substrate comprising, for example, a polyethylene terephthalate film and the like and a magnetic medium layer comprising ferromagnetic substance powder and binder formed thereon. As the ferromagnetic substance powders used for the coated type magnetic recording mediums, .gamma.-Fe.sub.2 O.sub.3, Co adherent .gamma.-Fe.sub.2 O.sub.3, CrO.sub.2, needle shaped magnetic substance powders such as metal Fe, and Ba ferrite having plate shaped crystals have been known. Particularly, with respect to the Ba ferrite particles, since the easily magnetizing axis is located perpendicular to the plate surface, by arranging the plate surface of the Ba ferrite particles in parallel with the substrate surface, a magnetic recording medium suitable for the vertical magnetic recording system which enables data to be recorded in a high density can be formed.
The recording resolution of the conventional surface oriented medium using needle shaped particles depends on the length of the longer axis of the needle shaped particles (normally, 0.2 to 0.4 .mu.m). On the other hand, the recording resolution of the medium where the Ba ferrite particles are surface-oriented depends on the plate thickness of the ferrite particles (normally, 0.005 to 0.03 .mu.m). Thus, the Ba ferrite which is used for the surface oriented recording medium allows the recording density to be remarkably improved.
As the Ba ferrites which satisfy the above conditions, M type Ba ferrite BaFe.sub.12 O.sub.19 and W type ferrite BaMe.sub.2 Fe.sub.16 O.sub.27 (where Me is a bivalent metal)
wherein the easily magnetizing axis is located perpendicular to the plate surface of the crystals have been known as well as types where the atoms of the M type or W type Ba ferrites are partially replaced with other atoms. These types of the magnetic recording mediums are produced in the following manner. The Ba ferrite magnetic powder described above is mixed with a synthetic resign type binder in a solvent. They are coated on the surface of a nonmagnetic substrate such as a polyethylene terephthalate film. The magnetization easy axis of the Ba ferrite in the coated layer is oriented in parallel with or perpendicular to the surface of the substrate and then dried.
However, to satisfactorily obtain a recording medium with the high recording density using the Ba ferrite magnetic ferrite powder, like the conventional surface oriented recording medium, it is desired to provide as higher orientation ratio and higher coercive force as possible. However, we found that although the high orientation ratio (squareness) and the high coercive force satisfy the necessary conditions, but does not satisfy sufficient conditions for improving the output through our long time study. Particularly, in the short wave band, to obtain a high output, we found that it is necessary to control the ratio Hr(90.degree.) Hr(0.degree.) of the remanence coercive force Hr(0.degree.) in the oriented direction (0.degree.) and the remanence coercive force (90.degree.) perpendicular to the oriented direction with respect to the magnetic fine structure of the recording medium as well as increasing the coercive force Hc and the squareness of the recording medium and thereby made the present invention.
The remanence coercive force Hr will be described in the following. Hr(.theta.) represents the strength of the magnetic field where the magnetization of the magnetic powder is irreversibly reversed when the magnetic field is applied with an angle of .theta. against the easily magnetizing axis of the magnetic powder. According to our study, it becomes obvious that the remanence coercive force Hr(90.degree.) / Hr(0.degree.) closely relates to the aggregation state of the magnetic powder of the magnetic recording medium and the size of the magnetic powder particles. The value of the remanence coercive force can be controlled to a considerable extent by selecting the size of the magnetic powder particles, the dispersion time for dispersing the magnetic powder particles in binder, the rotation speed of the dispersing machine, the type of the dispersing agent, the orientation process conditions, and so forth.
The present invention was made according to the above fact we found. An object of the present invention is to provide a magnetic recording medium which allows a large reproduction output in a short wave band to be obtained in high density by properly controlling the coercive force of a Ba ferrite type magnetic medium, the squareness, and Hr(90.degree.) Hr(0.degree.), the magnetic recording medium being able to be easily produced.